Along with recent improvements on network services and high performance of mobile communication terminals, communication terminals are prevailing which can transmit and receive data by selecting a proper communication protocol (transmission/reception method) from a plurality of communication protocols.
Under such circumstances, a mobile communication terminal proposed in JP Laid-Open Gazette No.11-136751 has the functions as shown in the block diagram of FIG. 8.
Referring to FIG. 8, a memory unit 6 has a message storage area 6a, first and second code conversion table storage areas 6b and 6c. The message storage area 6a stores a message written by predetermined text codes converted from character data input from a key input unit 63. The first code conversion table storage area 6b is used for converting character data into numerical data to be transmitted to a pager. The second code conversion table storage area 6c is used for converting character data into data to be transmitted to a PHS terminal.
A control unit 5 has a personal computer communication control means 5a, a pager call control means 5b and a mobile station message communication control means 5c. The personal computer communication control means 5a controls the transmission of character data stored in the memory unit 6 as an email. The pager call control means 5b controls the transmission of character data to a pager by converting the character data by using the first code conversion table storage area 6b. The mobile station message communication control means 5c controls the transmission of character data to a PHS terminal by converting the character data by using the second code conversion table storage area 6c. 
Message data created by the mobile communication terminal is converted in accordance with the type of a communication destination terminal. The above-cited publication does not teach data conversion for terminals of the same type having a plurality of transmission methods.
A mobile communication terminal is known which has a plurality of different communication methods called “bearers” and can communicate by selectively using one of the methods.
For example, such a mobile communication terminal has two different transmission methods, an SMS bearer and a DAS bearer. FIG. 2 shows sequences of data transmission/reception services using the SMS bearer and DAS bearer.
FIG. 2(a) shows a sequence of the SMS bearer (Sky Walker). During an SMS communication process, a wireless channel is established between a mobile communication terminal MS and a network NW, and after the SMS communication process is completed, the wireless channel is disconnected.
FIG. 2(b) shows a sequence of the DAS bearer (J—Sky Walker). During a DAS communication process, a non-voice channel is established between a mobile communication terminal MS and a network NW, and after the DAS communication process is completed, the non-voice channel is disconnected.
The largest difference between the SMS and DAS bearers in terms of an NW access sequence is whether or not the non-voice channel is used. In the SMS bearer, the non-voice channel is not used, but data transmission/reception is performed by placing data on a message at the L3 level stipulated in RCR-STD27. In the DAS bearer, data transmission/reception is performed by using the non-voice channel.
The DAS bearer has a larger data size capable of transmission/reception of data of one session than that of the SMS bearer, and can attach a file. The SMS bearer can set only one message data destination site, whereas the DAS bearer can set five message data destination sites at the maximum. However, the communication cost of the DAS bearer becomes correspondingly higher than that of the SMS bearer.
A conventional mobile communication terminal is required to determine which bearer is used before the edition of message data. The bearer once determined cannot be changed during or after the edition of message data.
The length of message data is variable so that there is a high possibility that the length of message data may become longer or shorter during the edition. Therefore, for example, if the SMS bearer is selected and the length of message data becomes longer than expected and exceeds 128 bytes, then the message data once created cannot be used and it becomes necessary to create new message data.
Conversely, if the DAS bearer is selected and the length of message data becomes shorter than expected and shortens smaller than 128 bytes, then the communication cost of the higher DAS bearer fee is incurred, although the SMS bearer fee can otherwise be used.
The present invention has been made considering the technical disadvantages in the prior art as stated above, and has two main objectives.
A first objective of the present invention is to provide a mobile communication terminal capable of selecting or changing a transmission method for transmitting message data to a destination terminal of the same kind, during or after the edition of message data.
A second objective of the present invention is to provide a mobile communication terminal capable of automatically judging a transmission method for transmitting message data to a destination terminal of the same kind, after the edition of message data.